Boreholes are drilled into the Earth's formation to recover deposits of hydrocarbons and other desirable materials trapped in the formations below. Typically, a well is drilled by connecting a drill bit to the lower end of a series of coupled sections of tubular pipe known as a drillstring. Drilling fluids, or mud, are pumped down through a central bore of the drillstring and exit through ports located at the drill bit. The drilling fluids act to lubricate and cool the drill bit, to carry cuttings back to the surface, and to establish sufficient hydrostatic “head” to prevent formation fluids from “blowing out” the borehole once they are reached. When the borehole is drilled deep enough to reach a point of interest, operations to perforate and fracture the subterranean formation are performed to enable hydrocarbons, if present, to flow from the formation into the newly drilled borehole. Because the hydrostatic pressure of the column of drilling mud can be higher than the reservoir pressures of the hydrocarbons, the hydrocarbons may not flow from the formation into the borehole on their own. Before full-scale recovery operations are commenced, drilling and production operators prefer to test the formation fluids to ensure the proper type and quantity of hydrocarbons are present in the formation before completing the well. Once the formation fluids are properly identified, various operations to retrieve the hydrocarbons therein will be performed.
To test the fluids, a formation tester is typically deployed downhole. Various formation fluid testers for wireline and logging-while-drill applications are known in the art, including the modular dynamic tester sold under the trade name of MDT™ by Schlumberger Technology Corp. (Houston, Tex.). Detailed description of these tools may be found in U.S. Pat. Nos. 4,860,581 and 4,936,139 issued to Zimmerman et al. and U.S. Published Patent Application No. 2004/0104341, by Betancourt et al. These patents and application are assigned to the assignee of the present application and are incorporated by reference in their entireties.
FIG. 1 illustrates a schematic of a formation tester 10 suspended in the borehole 12 from the lower end of a typical multiconductor cable 15 that is spooled in a usual fashion on a suitable winch (not shown) on the formation surface. The cable 15 is electrically coupled to an electrical control system 18 on the formation surface. The tool 10 includes an elongated body 19 which encloses the downhole portion of the tool control system 16. The elongated body 19 also carries a selectively extendable fluid admitting assembly 20 and a selectively extendable tool anchoring member 21 which are respectively arranged on opposite sides of the tool body. The fluid admitting assembly 20 is equipped for selectively sealing off or isolating selected portions of the wall of the borehole 12 such that pressure or fluid communication with the adjacent earth formation 14 is established. Also included with tool 10 are means for determining the downhole pressure and temperature (not shown) and a fluid analysis module 25 through which the fluid flows. The fluid may thereafter be expelled through a port (not shown), or it may be sent to one or more fluid collecting chambers 22 and 23, which may receive and retain the fluids obtained from the formation. Control of the fluid admitting assembly, the fluid analysis section, and the flow path to the collecting chambers is maintained by the electrical control systems 16 and 18. As will be appreciated by those skilled in the art, the electrical control systems may include one or more microprocessors, associated memory, and other hardware and/or software to implement the invention.
Before formation samples are collected into collecting chambers 22 and 23, it is desirable to be certain that the fluids are from the virgin formation, i.e., not contaminated by drilling fluid from the invaded zone. To ensure that virgin formation fluids are collected, a fluid analyzer 25 is used to monitor the properties of the fluids while they are being drawn. The fluid analysis module 25 may be an optical module, a pressure sensor module, a resistivity module, or the like. Among these, the resistivity module is particularly useful because of its wide dynamic range. A typical resistivity module may include several electrodes that are in contact with the fluid. These electrodes are used to inject currents into the fluid and to measure the voltage drop over a distance. An example of such a module is disclosed in FIG. 1 (item 56) of U.S. Pat. No. 4,860,581, issued to Zimmerman. FIG. 2 shows one example of such a module (sensor).
As shown in FIG. 2, the fluid resistivity is determined by a four electrode sensor, where the four electrodes are short metal tubes separated from each other and from the input and output flow lines by short insulating tubes. The two outermost electrodes inject an electrical current (I) into the fluid sample, while the voltage drop (V) between the two innermost electrodes is measured. With a known current (I) and the measured voltage (V), the resistivity of the fluid is obtained.
However, these electrode devices are exposed to the fluids in the flow line that can be relatively high pressures (up to 30,000 psi). Therefore, good seals (e.g., bulkhead, o-rings or other mechanical seals) are necessary to protect the electronic parts that are outside the flow line and are at atmosphere pressure (about 14 psi). As boreholes drilled at such depths are often at the smallest gauge diameter, such measurement equipment and the sealing mechanisms (bulkhead and o-rings) are necessarily of a very small form factor. In the limited volume available for the resistivity sensor, it is difficult to achieve pressure seals between all of the insulated tubes and metal tubes. For a sensor shown in FIG. 2, at least eight seals would be needed; ten are needed including the seals between the outermost insulating tubes and the input and output fluid lines. Instead, four bulkhead electrical feed-throughs are used for the four wires connecting the electrodes to the electronics. At extreme temperatures and pressures, even the four bulkhead feed-throughs can be unreliable. As a result, there is great difficulty in producing a reliable resistivity sensor.
Therefore, there still exists a need for methods and apparatus for resistivity measurement that may be reliably used in formation testers or similar downhole equipment.